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A Hologram is recorded by the interference of two coherent wavefronts. When a hologram is illuminated by one of the beams used in its construction process, the light diffracted by the hologram reconstructs the other wavefront completely, including phase and amplitude information. As such, if one of the wavefronts is originated from a three-dimensional object, an image of this 3D object can be reconstructed from its hologram. The term xe2x80x9chologramxe2x80x9d is not only used to describe a device that reproduces the image of a 3D object, it is now commonly used to describe any device that can diffract light into a multitude of colors. These diffractive devices are used in graphic design for wrapping papers, package covers, labels to authenticate products and many other applications. It is difficult to pinpoint when the commercialization of such holograms began. In searching patent literature, an early U.S. Pat. No. 3,567,561 issued in 1971 described the use of composite grating structures as surface ornaments. Holograms displayed on credit cards are probably the earliest commercial holograms used on a large scale.
There are many methods for producing holograms. The 3D holograms on credit cards are called rainbow holograms (see U.S. Pat. No. 3,633,989). This type of hologram can produce 3D images with only horizontal parallax and can be reconstructed with a white light source. Ornamental surface type holograms are made by the interference of two parallel laser beams or two diverging laser beams with the same divergence cone. When a graphic pattern is composed of many hologram segments having different angles of rotation and different periods, each segment in the pattern has to be recorded sequentially on the same recording surface. For this reason a technique similar to a dot matrix printer was developed and used grating dots to construct graphic patterns. A patent was granted in Taiwan (Taiwan Patent 263565 issued in 1984) for one such system (see also U.S. Pat. No. 6,043,913). This Taiwan patent could be one of the earlier patents describing a dot matrix grating system for producing holograms. FIG. 1 shows the optical system for recording a dot matrix grating according to Taiwan Patent 263565. An incoming laser beam 101 is split into two beams 105 and 106 by beam splitter 104. These two beams are recombined at recording plane 108. Since only one lens 102 is used to focus the laser beam on surface 108, only one beam, 105 or 106, can be focused perfectly on surface 108. As a result, the diameter of the focused spots on surface 108 have to be sufficiently large so that both focused beams are within the depth of focus of lens 102. The period of the fringes within the overlapping beams is given by   T  =            λ              sin        ⁢                  xe2x80x83                ⁢        ϑ              .  
The fringe period can be adjusted by changing the angle of the prism mirror 107. The orientation of the interference fringes is set by rotating the optical assembly consisting of prism 104 and prism mirror 107. There are a number of problems related to this early design:
(1) The required depth of focus results in a very large beam spot on the recording surface,
(2) The laser used in this system must have long coherent length because the optical path length of the two beams are not equal,
(3) The beams on the recording plane are circular in shape with non-uniform beam profiles.
(4) The fringes are not continuous across adjacent grating dots.
For these reasons, the resolution of the early dot matrix system was limited to about 400 dots per inch and not very efficient in diffracting light.
FIG. 2 shows a more recent system for recording dot matrix holograms. A laser beam 201 is directed by a mirror 202 to a beam splitter 203, with output beams 204 and 205. Beam 205 is directed to a prism mirror 206. This system uses additional prism mirrors 207, 208, 209 and 210 to equalize the optical path length to reduce the coherence requirement of the laser source. A lens 213 is also used to simultaneously focus beams 204 and 205 on the recording surface 214. The spot diameter on the recording surface is given by xcex4=xcexF/d, where xcex is the wavelength of the laser light, F is the focal length of lens 213 and d is the diameter of the laser beams. Suppose that a spot diameter xcex4=10 xcexcm is needed for the system and xcex=0.5 xcexcm, the ratio of   F  d
is equal to 20. The period of the fringes is equal to       T    =          λ              2        ⁢        sin        ⁢                  xe2x80x83                ⁢        θ              ,
because both beams subtend an angle xcex8 with respect to the optical axis. In FIG. 2 it can be seen that the focal length F and the diameter of the lens 213 is also related by       tan    ⁢          xe2x80x83        ⁢    θ    =            D              2        ⁢        F              .  
To obtain T=1 xcexcm , the diffraction angle is equal to 14.5 degree. This angle determines that the lens 213 must be an f-2 lens. In this dot matrix system, prisms 211 and 212 can be moved up and down in unison to change the interference angle xcex8 and hence the period of the fringes. In spite of the improvements in this more recent system over the system shown in FIG. 1, the problems related to beam shape, beam non-uniformity, and fringe continuity remained unsolved.
U.S. Pat. No. 5,291,317 proposed an optical system, which further resolved some of the aforementioned difficulties. FIG. 3 shows the optical system according to U.S. Pat. No. 5,291,317. A laser beam 301 illuminates a mask 302 and a grating 303. The lens 307 produces a de-magnified image of the grating on the, recording surface 308. The mask 302 defines an aperture so that the shape of the grating dot on the recording surface 308 can be rectangular, hexagonal or circular in shape. The laser beam 301 has been expanded so that its intensity profile, between its perimeters 304 and 305, on the grating 303 is nearly uniform. The grating is mounted on a rotary stage so that its fringes can be rotated under computer control. This system is simple in concept but with a fundamental optical restriction on the lens 307. Suppose that the lens 307 has focal length F=10 mm and it is used to de-magnify the grating image by a factor 10. In order to record a grating dot with a fringe spacing of 1 xcexcm, with a laser wavelength of xcex=0.5 xcexcm the period of the grating 303 is 10 xcexcm. The diffraction angle of this grating according to relationship sin xcex8=xcex/T is equal to 2.86 degree. To achieve the 10xc3x97 reduction, the grating is approximately 100 mm from the lens. Therefore, 1st order beam will be at a distance 5 mm from the center of the lens 307. This means that ratio of the focal length to the diameter of the lens (f-numer) is about 1. This lens is difficult to design, if not impractical. This difficulty can not be avoided by using smaller de-magnification. For example, reducing the de-magnification to 5 will reduce the distance between the grating 303 and the lens 307 to 50 mm. However, the diffraction angle will increase from 2.86 degrees to 5.74 degrees. The result is still that we need a lens aperture equal to the focal length of the lens. However, when a laser is used as the light source and a spatial filter 306 is used to block the 0th order wave from the grating 303, the intensity variation on plane 308 has twice the spatial frequency of grating 303. This phenomenon can be explained as follows. Suppose that the complex amplitude of the phase grating image on the recording plane is given by       i    ⁡          (      x      )        =      1    +                  sin        ⁡                  (                                    2              ⁢              π              ⁢                              xe2x80x83                            ⁢              x                        T                    )                    .      
After the aperture 306 stops the 0th order of grating 303, the intensity variation on the recording plane 308 is equal to       I    ⁡          (      x      )        =                    [                  sin          ⁡                      (                                          2                ⁢                π                ⁢                                  xe2x80x83                                ⁢                x                            T                        )                          ]            2        =                  1        2            ⁡              [                  1          +                      cos            ⁡                          (                                                4                  ⁢                  π                  ⁢                                      xe2x80x83                                    ⁢                  x                                T                            )                                      ]            
Because of this coherent effect, the system as described in FIG. 3 can use an f-2 lens instead of an f-1 lens to record grating dot with 1 xcexcm fringes. The fringes in adjacent grating dots produced by this system are still not contiguous.
Another method for producing grating patterns is shown in U.S. Pat. Nos. 4,510,575 and 6,268,893 show grating or hologram images being put on a display screen, with optics being used to reduce the image and focus it onto a recording substrate. The ""575 patent shows a microscope reducing an image from a CRT. The ""893 patent shows a laser beam being projected through a LCD (Liquid Crystal Display), which is then imaged onto a recording material.
This invention describes a novel dot matrix system, which uses an electronic display panel as a diffractive optical device to produce two laser beams. The interference pattern of these two beams, at the focal plane of a lens, forms a grating spot with a shape and beam profile determined by the wavefronts diffracted by the electronic display device.
The invention uniquely combines aspects of the interfering beam prior art and the prior art using an imaging display. Instead of simply reducing a diffraction image on a display device as in the imaging display prior art, the invention puts two different Fourier transforms on the display, the laser beam interacts with these Fourier transforms, and a lens focuses the two beams on the recording medium, with the two beams interfering with each other to produce the desired grating pattern. Because multiple orders of the wavefront will be produced by the display device, a light blocking element is used, with an aperture to pass only the desired order(s) of the beams.
In one embodiment, the display device is a LCD. The laser can be projected onto the LCD through a beamsplitter from the front, then reflected back through the beamsplitter, aperture and lens to the recording medium. Alternately, the laser beam can be directed through the LCD from behind. By controlling the display, the two wavefronts are produced, with the middle portion of the laser beam being blocked, to result in two distinct beams emanating from the LCD. In one embodiment, the LCD modulates the amplitude of the laser wavefront, in other embodiment it modulates the phase.
The position of the recording medium relative to the optics can be moved after each spot, generating a grid of spots, each spot having its own grating pattern. Another feature of the present invention allows the fringes making up the grating for a spot to be slightly altered so that they line up with the fringes of an adjacent spot, making the fringes contiguous across the adjacent boundary.